JP6380854B2 - Negative electrode active material for sodium ion secondary battery, method for producing the same, and sodium ion secondary battery - Google Patents

Description

  The present application relates to a negative electrode active material for a sodium ion secondary battery, a manufacturing method thereof, and a sodium ion secondary battery.

  In recent years, the demand for secondary batteries has increased as a power source for transportation equipment such as electric vehicles, hybrid vehicles, and plug-in hybrid vehicles, and large-sized power storage devices for home use and commercial use. Lithium secondary batteries are widely used as these power sources. Lithium secondary batteries use lithium ions as charge carriers. However, lithium is a rare metal, and in addition to being expensive, there is a problem that the output is small.

  As a new secondary battery, research on a sodium ion secondary battery has been conducted. Sodium ion secondary batteries use sodium ions as charge carriers. Sodium is more abundant than lithium and is available at a low price. Therefore, sodium is drawing attention as a secondary battery that can be manufactured at low cost and can be increased in size. However, conventionally, a material that can be used as a negative electrode active material of a lithium secondary battery (for example, a carbon material having a high degree of graphitization such as graphite) is used as it is as a negative electrode active material of a sodium ion secondary battery. Even so, it has been found that it is very difficult to realize a sodium ion secondary battery having sufficient performance (see Patent Document 1). For this reason, positive and negative electrode materials, particularly high-capacity negative electrode materials, have been demanded and developed for practical use of sodium ion secondary batteries.

  For example, Patent Document 1 proposes using an amorphous glassy carbon material as a negative electrode active material of a sodium ion secondary battery. Thereby, it is reported that a discharge capacity density per unit weight of 265 mAh / g can be obtained at the maximum.

  Patent Document 2 describes that hard carbon is used as a negative electrode active material in a sodium ion secondary battery using a non-aqueous electrolyte containing a specific electrolyte additive. Thus, it has been reported that a discharge capacity density per unit weight of about 250 mAh / g at maximum can be obtained.

International Publication No. 2009/066955 International Publication No. 2010/109889

  In a conventional sodium ion secondary battery, a negative electrode active material having a higher discharge capacity per unit volume has been demanded.

  One non-limiting exemplary embodiment of the present application provides a negative electrode active material for a sodium ion secondary battery having a higher discharge capacity per unit volume, a method for producing the same, and a sodium ion secondary battery using the same.

  In order to solve the above-described problem, one aspect of the present invention is the porous carbon material having a plurality of openings communicating to the surface, a plurality of closed holes not communicating to the surface, and a solid portion made of a carbon material. The distance between the (002) planes of the solid part is 0.340 nm or more and 0.410 nm or less, and the plurality of openings, the plurality of closed holes, and the plurality of closed holes with respect to the volume sum of the solid part Sodium ion having a volume ratio of 0% or more and 10% or less, and a volume ratio of the plurality of openings to the volume sum of the plurality of openings, the plurality of closed holes, and the solid portion is 0% or more and 50% or less Includes a negative electrode active material for a secondary battery.

  According to one embodiment of the present invention, a negative electrode active material for a sodium ion secondary battery having a large capacity per unit volume is provided. In addition, a high-capacity sodium ion secondary battery can be realized.

It is sectional drawing which shows the negative electrode active material for sodium ion secondary batteries of exemplary embodiment. It is sectional drawing which shows the sodium ion secondary battery of exemplary embodiment. It is a graph which shows the relationship between the first time discharge capacity per volume of the sodium ion secondary battery of Examples 1-7, Comparative Example 1, and Comparative Example 6, and the aperture ratio of a negative electrode active material.

  The inventor of the present application has examined the techniques disclosed in Patent Document 1 and Patent Document 2 in detail. In Patent Documents 1 and 2, glassy carbon and hard carbon as a negative electrode active material for a sodium ion secondary battery have a problem of low charge / discharge capacity density per volume. In particular, the hard carbon described in Patent Document 2 has a small apparent density of 2 g / cc or less, so that a sufficient discharge capacity per unit volume cannot be obtained.

  Further, Patent Documents 1 and 2 describe that glassy carbon and hard carbon can be used as the negative electrode active material of the sodium ion secondary battery. It is not disclosed whether such a carbon structure affects the charge / discharge capacity of sodium.

  The inventors of the present application paid attention to carbon materials as a negative electrode active material for sodium ion secondary batteries, and examined the structural analysis of various carbon materials and the reactivity between these carbon materials and sodium ions. As a result, it has been found that a carbon material having a specific structure exhibits a larger capacity density per volume than the conventional one as a negative electrode active material for a sodium ion secondary battery.

  The outline of one embodiment of the present invention is as follows.

  The negative electrode active material for a sodium ion secondary battery according to one aspect of the present invention includes a plurality of pores that communicate with the surface, a plurality of pores that do not communicate with the surface, and a solid portion made of a carbon material. A distance between (002) planes of the solid portion is 0.340 nm or more and 0.410 nm or less, and the plurality of openings, the plurality of closed holes, and the plurality of the solid portions with respect to a volume sum of the solid portions The volume ratio of the closed holes is 0% or more and 10% or less, and the volume ratio of the plurality of openings to the volume sum of the plurality of openings, the plurality of closed holes, and the solid portion is 0% or more and 50% or less. is there.

  The distance between the (002) planes of the solid part is, for example, 0.36 nm or more.

  The volume ratio of the plurality of openings to the volume sum of the plurality of openings, the plurality of closed holes, and the solid portion is, for example, 20% or less.

  A sodium ion secondary battery of one embodiment of the present invention includes a negative electrode including the above negative electrode active material, a positive electrode including a positive electrode active material capable of occluding and releasing sodium ions, and an electrolyte including sodium ions.

  A method for producing a negative electrode active material for a sodium ion secondary battery according to one embodiment of the present invention includes a step of preparing an organic material or a porous carbon material that serves as a carbon source, and the inertness of the organic material or the porous carbon material. A step of obtaining a porous carbon material by heat treatment in an atmosphere, wherein the porous carbon material comprises a plurality of apertures communicating with the surface, a plurality of closed pores not communicating with the surface, and a carbon material. A distance between carbon (002) planes in at least a part of the solid part is 0.340 nm or more and 0.410 nm or less, the plurality of openings, the plurality of closures, and the solid part The volume ratio of the plurality of closed holes to the volume sum of the solid part is 0% or more and 10% or less, and the volumes of the plurality of openings, the plurality of closed holes, and the volume of the plurality of holes to the volume sum of the solid part Ratio % Or more is 50% or less.

  The organic material is, for example, a cellulose resin. 1100 degreeC or more and 1300 degrees C or less may be sufficient as the said heat processing temperature, for example.

  The organic material is, for example, a phenolic resin. The heat treatment temperature may be, for example, 1000 ° C. or higher and 1300 ° C. or lower.

  The organic material is, for example, phenolphthalein. The heat treatment temperature may be, for example, 1000 ° C. or higher and 1300 ° C. or lower.

  The porous carbon material is, for example, an activated carbon material. 1600 degreeC or more and 2500 degrees C or less may be sufficient as the said heat processing temperature, for example.

(Embodiment)
Hereinafter, embodiments of a negative electrode active material for a sodium ion secondary battery according to the present invention will be described with reference to the drawings.

  FIG. 1 is a schematic cross-sectional view illustrating the configuration of the negative electrode active material for a sodium ion secondary battery of this embodiment.

  The negative electrode active material for a sodium ion secondary battery according to the present embodiment includes a porous carbon material. The porous carbon material 10 includes a plurality of openings 12 that communicate with the surface 11 of the porous carbon material 10, a plurality of closed holes 13 that do not communicate with the surface 11, and a solid portion 14. In FIG. 1, one opening 12 and one closing hole 13 are schematically shown.

  The porous carbon material 10 may have various shapes as long as the porous carbon material 10 has the structure having the above-described opening 12, closing hole 13, and solid part 14, and various types generally used as an active material for a sodium ion secondary battery. The shape may be provided. Specifically, the porous carbon material 10 may have a particle shape, a flake shape, or a thin film shape. When the porous carbon material 10 has a particle shape, the average particle diameter is, for example, 0.01 μm or more and 100 μm or less, preferably 1 μm or more and 50 μm or less. When the average particle diameter is smaller than 1 μm, the surface activity is high and handling may be difficult. On the other hand, when larger than 50 micrometers, the reaction rate as a negative electrode active material may become slow.

  Moreover, the negative electrode active material for sodium ion secondary batteries should just contain the porous carbon material, and may contain the other negative electrode active material, an additive, etc. “Mainly” means containing 50% by weight or more of the whole. Preferably, the negative electrode active material for a sodium ion secondary battery contains a porous carbon material in a proportion of 70% by weight or more with respect to the whole.

  The solid part 14 is made of a carbon material. Here, the carbon material refers to various substances mainly containing carbon and having a structure with a carbon-carbon bond. It originates in the manufacturing method of the carbon material, and may contain a small amount of other elements such as hydrogen and nitrogen in addition to carbon, and there is a portion partially containing elements other than carbon in the entire carbon material. May be.

In the present embodiment, in the carbon material constituting at least a part of the solid part 14, the distance between the (002) planes of the carbon skeleton formed by the sp ₂ hybrid orbital is 0.340 nm or more. The volume ratio of the closed holes 13 is 10% or less, and the opening ratio is 50% or less. Thereby, the negative electrode active material for sodium ion secondary batteries of the present embodiment has a large charge / discharge capacity per unit volume.

  According to the detailed examination by the inventors of the present application, in the porous carbon material, the site functioning as the sodium occlusion / release site is considered to be the solid portion 14 and the closed hole 13 that does not communicate with the surface 11. That is, sodium is occluded in the solid portion 14 and the closed hole 13 during charging, and the occluded sodium is released to the outside during discharging.

  In order to obtain a sufficient amount of sodium to be occluded and released into the solid portion 14, the distance between the (002) planes of the solid portion 14 is 0.340 nm or more, and the (002) planes of a general amorphous carbon material It was found that the maximum value of the distance was preferably 0.410 nm or less.

  It is preferable that the volume ratio of the closed hole 13 to the volume sum of the open hole 12, the closed hole 13, and the solid portion 14 is 10% or less. As described above, sodium is occluded in the closed pores 13 and the solid portion 14 of the porous carbon material, so that the volume of the porous carbon material increases as the volume of these increases. However, in order to obtain a negative electrode active material having a large capacity density per volume, the solid part 14 is most important than the closed hole 13 that does not communicate with the surface 11. The solid portion 14 also increases the mass as the porous carbon material, whereas the closed hole 13 is a cavity, so that the mass does not increase even if the closed hole 13 is increased. In order to increase the charge / discharge capacity per unit volume, the volume ratio of the closed holes 13 is preferably small. When the volume ratio of the closed holes 13 is increased, the total volume of the closed holes 13 that are the particle volume of the negative electrode active material and the solid portion 14 is increased, so that the apparent density (g / cc) is decreased. Therefore, it is preferable to have the solid portion 14 larger than the closed hole 13 that does not communicate with the surface 11 in order to obtain a negative electrode active material having a large capacity density per volume. In particular, when the volume ratio of the closed hole 13 to the volume sum of the open hole 12, the closed hole 13, and the solid portion 14 is 10% or less, the negative electrode active for a sodium ion secondary battery having a larger charge / discharge capacity per unit volume. A substance can be realized. If the volume ratio of the closed holes 13 is reduced, the weight of the porous carbon material increases, and the charge / discharge capacity per unit weight may be reduced.

  On the other hand, it is considered that the opening 12 occludes solvated sodium ions by being in direct contact with the nonaqueous electrolytic solvent in the sodium ion secondary battery. However, when the solvated sodium ions are occluded in the opening 12, an irreversible reaction can be caused by reducing the solvent or the electrolyte. Therefore, it is considered that irreversible sodium occlusion and release is unlikely to occur in the opening 12. For this reason, it is preferable that the number of the apertures 12 is small, and the volume ratio of the apertures 12 to the volume sum of the apertures 12, the closed holes 13, and the solid portion 14 is preferably 0% or more and 50% or less. When the volume ratio of the opening 12 is 50% or less, sodium that becomes an irreversible capacity at the first charge / discharge can be suppressed, and a negative electrode active material having a large charge / discharge capacity can be provided. In particular, when the volume ratio of the apertures 12 is 20% or less, the irreversible capacity during the first charge / discharge is reduced, and a negative electrode active material having a very large charge / discharge capacity can be provided.

  According to the study by the inventors of the present application, the opening 12 and the closing hole 13 have a size inside the hole (a size of a cross section perpendicular to the direction in which the hole extends), in particular, as long as sodium atoms can be inserted. There is no particular limitation on the length, and it has not been confirmed that they have a significant effect on the charge / discharge capacity. Considering that the atomic radius of sodium is about 0.2 nm and the diameter of Na ions solvated with propylene carbonate is about 0.4 nm (calculated by the Stokes method), the cross-sections of the apertures 12 and 13 It is considered that the size (diameter) of approximately 0.4 nm or more is sufficient. Moreover, the diameters of helium, argon, and nitrogen are about 0.3 to 0.4 nm, and as described below, the volume ratio described above is defined by values obtained by measurement using these gases. To do. Therefore, the size of the inscribed circle in the cross section perpendicular to the longitudinal direction of the opening 12 and the closing hole 13 is considered to be 0.4 nm or more. On the other hand, when the cross section of the open hole 12 and the closed hole 13 is larger than several nanometers, a plurality of sodium atoms can be arranged in the cross section of the closed hole 13. However, actually, it is considered that sodium is not occluded in the closed holes 13 so that sodium atoms are arranged at a high density in the cross section of the closed holes 13 due to repulsion between sodium atoms. For this reason, considering the effective use of the space of the closed hole 13, the size of the inscribed circle in the cross section perpendicular to the longitudinal direction of the closed hole 13 is preferably about 100 nm or less. In addition, when the above-described gas is used, the volume of pores having a diameter of approximately 100 nm or less can be accurately measured.

Thus, in the negative electrode active material for a sodium ion secondary battery according to the present embodiment, carbon formed by sp ₂ hybrid orbitals in the carbon material constituting at least part of the solid portion 14 of the porous carbon material. When the distance between the (002) planes of the skeleton is 0.340 nm or more, it can function well as a sodium ion storage / release site. The stored sodium is reversibly released. For this reason, the negative electrode active material which can occlude-release sodium reversibly is realizable. Further, when the volume ratio of the closed holes 13 is 10% or less and the volume ratio of the open holes 12 is 50% or less, a negative electrode active material having a larger charge / discharge capacity per unit volume than that of the related art can be obtained.

  The (002) interplanar distance in the porous carbon material contained in the negative electrode active material for a sodium ion secondary battery of the present embodiment can be determined by X-ray diffraction measurement. An example of a specific measurement procedure will be described in the order of sample adjustment, measurement, and analysis. For sample preparation, for example, the porous carbon material is dried at 120 ° C. under vacuum for 2 hours. Next, 10 wt% of standard Si (NIST 640d) is weighed with respect to the porous carbon material, and the dried porous carbon material and standard Si are mixed in a mortar. Thereby, the sample for X-ray diffraction measurement can be prepared. For the measurement, for example, Cu-Kα rays can be used as an X-ray source. X-rays are generated at an output of a tube voltage of 40 kV and a tube current of 40 mA, and a diffraction line is detected by scanning the sample in the range of 20 degrees to 30 degrees (2θ) by the 2θ / θ method.

In the analysis, the measurement result is corrected using the standard Si (111) peak position by the method described in JIS R7651, and the carbon (002) peak observed around 23 to 26 degrees with respect to the obtained correction value. From the value (2θ), the distance between (002) planes of the carbon skeleton structure is obtained using the Bragg equation (d ₀₀₂ = λ / sin θ _c / 2). In the case of Cu-Kα rays, λ = 0.15419 nm.

  Depending on the porous carbon material, only a part of the solid part is graphitized by heat treatment or the like when producing the porous carbon material, so that two or more (002) peaks of carbon are present at around 23 to 26 degrees. May be observed. The sharp peak observed at around 26 degrees is a peak attributed to a partially graphitized solid part, and the broad peak observed at the lowest angle is a peak attributed to a solid part having lower crystallinity. is there. In order to function as a passage for sodium, it is preferable that the distance between the (002) planes of the carbon skeleton structure is 0.36 nm or more. Therefore, for a carbon material in which two (002) peaks of carbon appear, The broad peak observed on the low angle side is the (002) peak of carbon, and the (002) interplane distance can be determined from the peak value (2θ).

Further, the volume ratio of closed pores in the carbon material contained in the negative electrode active material for sodium ion secondary battery of the present embodiment (hereinafter, sometimes simply referred to as a closed pore ratio) can be obtained as follows. it can. First, the volume V _OP (cc / g) of the opening 12 per unit weight of the carbon material is determined by gas adsorption measurement. Also, the volume of the solid portion 14 and the closed pores 13 of the porous carbon material can be determined by density measurement, as the inverse of the apparent density d _He (g / cc) of the porous carbon material.

The volume ratio R _CP (%) of the closed hole is obtained by the following formula.
R _CP (%) = (1 / d _He −1 / 2.26) / (V _OP + 1 / d _He ) × 100
Here, the denominator (V _OP + 1 / d _He ) of the above formula is the sum of the volumes of the open holes 12, the closed holes 13 and the solid part 14 per 1 g of the carbon material, and the numerator (1 / d _He − 1 / 2.26) is a value obtained by subtracting the volume of the solid part (1 / 2.26)) from the sum of the volumes of the solid part 14 and the closed hole 13 per 1 g of the porous carbon material. The volume of is shown. By dividing the numerator by the denominator and converting the obtained value to 100 fraction, the volume ratio R _cp (%) of closed pores of the carbon material can be obtained. The volume (1 / 2.26) of the solid part 14 is calculated from the true density of carbon 2.26 g / cc.

An example of a specific method for measuring the volume ratio R _cp (%) of the closed hole is shown. For example, the volume V _OP (cc / g) of the opening 12 per unit weight of the carbon material is a carbon material dried under vacuum at 120 ° C. for 2 hours as a pretreatment, and argon or nitrogen gas is used as an adsorption gas species. The amount of adsorbed gas at a relative pressure of 0.99 can be obtained by a fully automatic gas adsorption amount measuring device. The fully automatic gas adsorption amount measuring device is used as a device for measuring the total pore volume (cc / g), but in this measurement, gas is not adsorbed to the closed pores. The volume V _OP (cc / g) is being measured.

The apparent density (g / cc) of the carbon material can be obtained by an ultra pycnometer using a carbon material dried at 120 ° C. under vacuum for 2 hours as a pretreatment and helium as a measurement gas. Further, the volume ratio of pores in the carbon material (hereinafter sometimes simply referred to as the pore ratio) R _OP can be calculated by the following equation using V _OP and d _He .
R _OP (%) = V _OP / (V _OP + 1 / d _He ) × 100

Further, the volume ratio of the pores in the negative electrode active material of the present invention can be calculated according to the formula of 100 × V _OP / (V _OP + 1 / d _He ) using V _OP and d _He similar to the above. .

  The carbon material contained in the negative electrode active material for a sodium ion secondary battery of the present embodiment can be obtained, for example, by firing an organic material or a porous carbon material serving as a carbon source in an inert atmosphere. As the carbon source, cellulose resin, phenol resin, phenolphthalein and the like are preferable. Cellulosic resin, phenolic resin, and phenolphthalein may be in any form such as fibrous or particulate, but considering processing into active material particles after firing, a particle shape of several μm to several tens of μm, or A short fiber shape is preferred.

  As an inexpensive cellulosic resin, charcoal, sawdust, paper, or the like can be used. The heat treatment temperature is preferably 1100 ° C to 1300 ° C. The firing atmosphere is not particularly limited as long as it is inert, but a gas such as nitrogen, argon, helium, or neon is preferably used.

  As the phenolic resin, a novolac resin, a resole resin, or the like can be used. The heat treatment temperature is preferably 1000 ° C to 1300 ° C. The firing atmosphere is not particularly limited as long as it is inert, but a gas such as nitrogen, argon, helium, or neon is preferably used.

  The heat treatment temperature of phenolphthalein is preferably 1000 ° C to 1300 ° C. The firing atmosphere is not particularly limited as long as it is inert, but a gas such as nitrogen, argon, helium, or neon is preferably used.

  When the porous carbon material needs to be pulverized for granulation or the like, the organic material or the porous carbon material is preferably pulverized before the heat treatment. When the porous carbon material obtained by the heat treatment is pulverized, there is a possibility that the structure of the porous carbon material changes and the closed pores are converted into open pores.

  As the porous carbon material, an activated carbon material is preferable. The shape of the activated carbon material may be fibrous, particle or the like. In consideration of processing into the active material particles after firing, it is desirable that the activated carbon material be in the form of particles or short fibers having a size of several μm to several tens of μm. As an inexpensive activated carbon material, steam activated charcoal can be used. The heat treatment temperature is preferably 1600 ° C. or higher and 2500 ° C. or lower. The firing atmosphere is not particularly limited as long as it is inert, but a gas such as nitrogen, argon, helium, or neon is preferably used.

(Embodiment 2)
An embodiment of a sodium ion secondary battery according to the present invention will be described. FIG. 2 is a schematic cross-sectional view illustrating the configuration of the sodium ion secondary battery of this embodiment.

  In the example illustrated in FIG. 2, the positive electrode 23 includes a positive electrode current collector 21 and a positive electrode mixture layer 22 that is formed on the positive electrode current collector 21 and includes a positive electrode active material. The negative electrode 26 includes a negative electrode current collector 24 and a negative electrode mixture layer 25 formed on the negative electrode current collector 24 and including a negative electrode active material. The positive electrode 23 and the negative electrode 26 are disposed so that the positive electrode mixture layer 22 and the negative electrode mixture layer 25 face each other with the separator 27 interposed therebetween, and constitute an electrode group. The electrode group is covered with an exterior 28.

  The negative electrode mixture layer 25 contains the negative electrode active material for sodium ion secondary batteries described in the first embodiment. In addition to the negative electrode active material, the negative electrode mixture layer 25 may contain a conductive additive, an ionic conductor, and / or a binder as necessary. When the conductive assistant, the ionic conductor, and the binder are not included, the negative electrode active material may be a thin film formed on the negative electrode current collector 24.

  As described in Embodiment 1, the negative electrode active material includes a porous carbon material having an opening 12 that communicates with the surface, a closed hole 13 that does not communicate with the surface, and a solid portion 14. The distance between the (002) planes of carbon in at least a part of the solid part is 0.340 nm or more. Further, the volume ratio of the closed holes 13 is 10% or less, and the volume ratio of the open holes 12 is 50% or less. As can be seen from the examples described later, the negative electrode active material containing such a porous carbon material has a higher capacity per unit volume than the conventional negative electrode active material for sodium ion secondary batteries. For this reason, according to the present embodiment, it is possible to realize a sodium ion secondary battery having a higher capacity per volume than in the past.

  Further, the negative electrode may further have a (002) inter-surface distance of the solid portion 14 of 0.36 nm or more, or a volume ratio of the apertures 12 of 20% or less. Such an active material has a higher charge / discharge capacity density than conventional negative electrode active materials for sodium ion secondary batteries. For this reason, according to the present embodiment, it is possible to realize a sodium ion secondary battery having a higher capacity per volume than in the past.

  Conductive aids and ionic conductors are used to reduce electrode resistance. Examples of the conductive assistant include carbon materials (carbon conductive assistant) such as carbon black, graphite, and acetylene black, and conductive polymer compounds such as polyaniline, polypyrrole, and polythiophene. Examples of the ionic conductor include gel electrolytes such as polymethyl methacrylate and polymethyl methacrylate, and solid electrolytes such as polyethylene oxide.

  A binder is used in order to improve the binding property of the material which comprises an electrode. Specific examples include polyvinylidene fluoride, vinylidene fluoride-hexafluoropropylene copolymer, vinylidene fluoride-tetrafluoroethylene copolymer, polytetrafluoroethylene, carboxymethylcellulose, polyacrylic acid, styrene-butadiene copolymer rubber, Examples include polypropylene, polyethylene, and polyimide.

  As the negative electrode current collector 24, a porous or non-porous sheet or film made of a metal material such as aluminum, stainless steel, nickel, copper, and alloys thereof can be used. Aluminum and its alloys have the advantage of being inexpensive and easy to thin. As the sheet or film, metal foil, mesh, or the like is used. For reducing the resistance value, providing a catalytic effect, and strengthening the bond between the negative electrode mixture layer 25 and the negative electrode current collector 24 by chemically or physically bonding the negative electrode mixture layer 25 and the negative electrode current collector 24. Alternatively, a carbon material such as carbon may be applied to the surface of the negative electrode current collector 24 as a conductive auxiliary material.

  The positive electrode mixture layer 22 contains a positive electrode active material capable of occluding and releasing sodium ions. In addition to the positive electrode active material, the positive electrode mixture layer 22 may contain a conductive additive, an ionic conductor and / or a binder as necessary.

The positive electrode active material is not particularly limited as long as it is a material that occludes and releases sodium ions. For example, sodium-containing transition metal oxides, transition metal fluorides, polyanions and fluorinated polyanion materials, and transition metals Examples thereof include sulfides. Specifically, as the sodium-containing transition metal oxide, Na _x Me ¹ _y O ₂ (0 <x ≦ ¹ , 0.95 ≦ y <1.05, Me ¹ is Fe, Mn, Ni, Co, Cr and Including at least one selected from the group consisting of Ti). As the transition metal fluorides, and the like can be used NaFeF _3, NaMnF ₃ and NaNiF _3. Polyanion and fluorinated polyanion materials include NaMe ² PO ₄ , Na ₃ Me ² ₂ (PO ₄ ) ₃ , Na ₄ Me ² ₃ (PO ₄ ) ₂ P ₂ O ₇ , Na ₂ Me ² PO ₄ F and Na ₃ Me. ² ₂ (PO ₄ ) ₂ F ₃ (Me ² includes at least one selected from the group consisting of Fe, Mn, Ni, Co, Ti, V, and Mo) can be used. Ni ₃ S ₂ , FeS _2, TiS _2, etc. can be used as the transition metal sulfide. Among these, the use of a Na-containing transition metal oxide has advantages that the production cost is low and the average discharge voltage is high. As the conductive assistant, the ionic conductor, and the binder, the same materials as those for the negative electrode mixture layer 15 can be used.

  As the positive electrode current collector 21, a porous or non-porous sheet or film made of a metal material such as aluminum, stainless steel, titanium, and alloys thereof can be used. Aluminum and its alloys have advantages such as low cost and easy thinning. As the sheet or film, metal foil, mesh, or the like is used. For reducing the resistance value, imparting a catalytic effect, and strengthening the bond between the positive electrode mixture layer 22 and the positive electrode current collector 21 by chemically or physically bonding the positive electrode mixture layer 22 and the positive electrode current collector 21. Alternatively, a carbon material such as carbon may be applied to the surface of the positive electrode current collector 21 as a conductive auxiliary material.

  As the separator 27, a porous film made of polyethylene, polypropylene, glass, cellulose, ceramics, or the like is used, and the pores are impregnated with an electrolyte.

  Examples of the nonaqueous electrolyte used in the battery include a nonaqueous solvent containing a sodium salt, a gel electrolyte, or a solid electrolyte.

Examples of the sodium salt include sodium hexafluorophosphate (NaPF ₆ ), sodium tetrafluoroborate (NaBF ₄ ), sodium perchlorate (NaClO ₄ ), sodium bistrifluoromethylsulfonylimide (NaN (SO ₂ CF ₃ ) ₂ ), sodium bisperfluoroethylsulfonylimide (NaN (SO ₂ C ₂ F ₅ ) ₂ ), sodium bisfluoromethylsulfonylimide (NaN (SO ₂ F) ₂ ), NaAsF ₆ , NaCF ₃ SO ₃ or difluoro (Oxalato) sodium borate and the like can be used. From the viewpoint of battery safety, thermal stability, and ion conductivity, NaPF ₆ is preferably used. In addition, you may use 1 type in the said electrolyte salt, and may use it in combination of 2 or more type.

  Examples of the non-aqueous solvent include cyclic carbonates, chain carbonates, esters, cyclic ethers, chain ethers, nitriles, amides and the like that are usually used as non-aqueous solvents for batteries. One of these solvents may be used alone, or two or more thereof may be used in combination.

  Examples of the cyclic carbonate include ethylene carbonate, propylene carbonate, butylene carbonate, etc., and those in which some or all of these hydrogen groups are fluorinated can be used. For example, trifluoropropylene carbonate, fluoro Examples include ethyl carbonate.

  Examples of chain carbonic acid esters include dimethyl carbonate, ethyl methyl carbonate, diethyl carbonate, methyl propyl carbonate, ethyl propyl carbonate, methyl isopropyl carbonate, and the like, and some of these hydrogen groups are fluorinated. It is possible to use.

  Examples of the esters include methyl acetate, ethyl acetate, propyl acetate, methyl propionate, ethyl propionate, and γ-butyrolactone. Examples of cyclic ethers include 1,3-dioxolane, 4-methyl-1,3-dioxolane, tetrahydrofuran, 2-methyltetrahydrofuran, propylene oxide, 1,2-butylene oxide, 1,4-dioxane, 1,3,5. -Trioxane, furan, 2-methylfuran, 1,8-cineol, crown ether and the like.

  As chain ethers, 1,2-dimethoxyethane, diethyl ether, dipropyl ether, diisopropyl ether, dibutyl ether, dihexyl ether, ethyl vinyl ether, butyl vinyl ether, methyl phenyl ether, ethyl phenyl ether, butyl phenyl ether, pentyl phenyl Ether, methoxytoluene, benzyl ethyl ether, diphenyl ether, dibenzyl ether, o-dimethoxybenzene, 1,2-diethoxyethane, 1,2-dibutoxyethane, diethylene glycol dimethyl ether, diethylene glycol diethyl ether, diethylene glycol dibutyl ether, 1,1 -Dimethoxymethane, 1,1-diethoxyethane, triethylene glycol dimethyl ether, tetraethy Such as glycol dimethyl and the like.

  Nitriles include acetonitrile and the like, and amides include dimethylformamide and the like.

<Examples and Comparative Examples>
As examples and comparative examples, a negative electrode active material evaluation cell for a sodium ion battery having a negative electrode active material as a test electrode and a sodium metal as a counter electrode was prepared and evaluated. The method and results will be described below. In addition, embodiment of this invention is not limited to the Example demonstrated below.

1. Cell fabrication (Example 1)
Production of Negative Electrode Active Material A carbon material to be a negative electrode active material was produced by the following three steps: a carbonization process, a classification process, and a heat treatment process.

  First, the carbonization process will be described. Using α-cellulose (Whatman quantitative filter paper (No. 40)) as a carbon source, the temperature was raised from room temperature at a rate of 10 ° C. per minute in a tubular furnace (Ar gas flow rate 1 L / min) in an Ar atmosphere. It heated until it reached | attained 1000 degreeC, and hold | maintained at 1000 degreeC for 1 hour. Thereafter, the heating was stopped, and the carbide was taken out from the tubular furnace after natural cooling.

  Next, the classification process will be described. The carbide obtained in the carbonization step was pulverized in an agate mortar and classified using a SUS standard sieve having an opening of 40 μm to obtain carbon powder.

  Finally, the heat treatment process will be described. Using the above-mentioned carbon powder, the temperature was raised from room temperature at a rate of 10 ° C. per minute in an annular furnace under an Ar atmosphere (Ar gas flow rate 1 L / min), and heated to reach 1200 ° C. for 1 hour at 1200 ° C. Retained. Thereafter, heating was stopped, and after natural cooling, the carbon material was taken out from the tubular furnace to obtain a negative electrode active material made of a porous carbon material.

Preparation of Sodium Ion Secondary Battery A test electrode was prepared using the carbon material prepared by the above method as a negative electrode active material and a copper foil as a current collector. The porous carbon material as a negative electrode active material and polyvinylidene fluoride as a binder were weighed to a weight ratio of 9: 1, and dispersed in an NMP solvent to obtain a slurry. The obtained slurry was coated on a copper foil using a coating machine. The coated electrode plate was rolled with a rolling mill, punched into a square with a side of 20 mm, and processed into an electrode state to obtain a test electrode.

  Next, a sodium ion secondary battery (evaluation cell) using sodium metal as a counter electrode was produced using the test electrode.

The preparation of the electrolyte solution and the evaluation cell were performed in a glove box in an Ar atmosphere having a dew point of −60 degrees or less and an oxygen value of 1 ppm or less. As the electrolytic solution, one obtained by dissolving 1 molar sodium hexafluorophosphate (NaPF ₆ ) in a solvent in which ethylene carbonate and diethyl carbonate were mixed at a volume ratio of 1: 1 was used. Moreover, sodium metal was crimped | bonded to the square nickel mesh whose side is 20 mm, and it was set as the counter electrode.

  The test electrode and the counter electrode were accommodated in the exterior in a state of facing each other through a polyethylene microporous membrane separator impregnated with an electrolytic solution, the exterior was sealed, and an evaluation cell was obtained.

(Example 2)
A negative electrode active material was prepared in the same manner as in Example 1 except that the temperature of the heat treatment step of the carbon source and the carbon material was different, and an evaluation cell was prepared in the same manner as in Example 1. A novolac resin was used as the carbon source, and the heat treatment temperature was 1000 ° C.

(Example 3)
Except that the temperature of the heat treatment step of the carbon material is different, the negative electrode active material was prepared by the same method as in Example 2, and the evaluation cell was prepared by the same method as in Example 2. The heat treatment temperature was 1200 ° C.

Example 4
A negative electrode active material was prepared in the same manner as in Example 1 except that the temperature of the heat treatment step of the carbon source and the carbon material was different, and an evaluation cell was prepared in the same manner as in Example 1. An activated carbon material (specific surface area 2600 m ² / g, average particle diameter 5 μm) was used as a carbon source, and the heat treatment temperature was 2100 ° C.

(Example 5)
A negative electrode active material was prepared in the same manner as in Example 4 except that the temperature of the heat treatment step of the carbon material was different, and an evaluation cell was prepared in the same manner as in Example 4. The heat treatment temperature was 2400 ° C.

(Example 6)
A negative electrode active material was prepared in the same manner as in Example 1 except that the temperature of the heat treatment step of the carbon source and the carbon material was different, and an evaluation cell was prepared in the same manner as in Example 1. Phenolphthalein was used as the carbon source, and the heat treatment temperature was 1000 ° C.

(Example 7)
A negative electrode active material was prepared in the same manner as in Example 6 except that the temperature of the heat treatment step of the carbon material was different, and an evaluation cell was prepared in the same manner as in Example 6. The heat treatment temperature was 1200 ° C.

(Comparative Example 1)
Except that the temperature of the heat treatment step of the carbon material is different, a negative electrode active material was prepared by the same method as in Example 1, and an evaluation cell was prepared by the same method as in Example 1. The heat treatment temperature was 1000 ° C.

(Comparative Example 2)
An evaluation cell was produced in the same manner as in Example 1 except that a hard carbon carbon material (Carbotron P, manufactured by Kureha Battery Materials Japan) was used as the negative electrode active material.

(Comparative Example 3)
Except that the temperature of the heat treatment step of the carbon material is different, a negative electrode active material was prepared by the same method as in Example 1, and an evaluation cell was prepared by the same method as in Example 1. The heat treatment temperature was 1800 ° C.

(Comparative Example 4)
An evaluation cell was prepared in the same manner as in Example 1 except that a graphite-based carbon material (NG12, manufactured by Kansai Thermochemical Co., Ltd.) was used as the negative electrode active material.

(Comparative Example 5)
Except that the temperature of the heat treatment step of the carbon material is different, the negative electrode active material was prepared by the same method as in Example 2, and the evaluation cell was prepared by the same method as in Example 2. The heat treatment temperature was 1500 ° C.

(Comparative Example 6)
A negative electrode active material was prepared in the same manner as in Example 4 except that the temperature of the heat treatment step of the carbon material was different, and an evaluation cell was prepared in the same manner as in Example 4. The heat treatment temperature was 1500 ° C.

(Comparative Example 7)
A negative electrode active material was prepared in the same manner as in Example 6 except that the temperature of the heat treatment step of the carbon material was different, and an evaluation cell was prepared in the same manner as in Example 6. The heat treatment temperature was 1400 ° C.

2. Characteristic evaluation
(A) Measurement of pore ratio, pore ratio, and interlayer distance of porous carbon material Examples 1-7 and pore ratios, pore ratios, and interlayer distances of the porous carbon materials of Comparative Examples 1-7 Was measured. The opening ratio and the closing ratio were measured by the following procedure.

Using a fully automatic gas adsorption amount measuring device (AS1-MP-9 manufactured by Quantachrome), gas adsorption measurement of a porous carbon material using argon was performed, and the porous carbon material was determined from the amount of adsorbed gas at a relative pressure of 0.99. The total pore volume V _OP (cc / g) of was determined.

Using an ultra pycnometer (Quantachrome Corp. Ultrapic1200e), using a helium measurement gas to obtain the apparent density d _{the He} of porous carbon material.

Further, from V _OP and d _He obtained from the results of gas adsorption measurement and density measurement, the pore ratio and the pore ratio of the negative electrode active material were determined according to the following formula.
Closed hole ratio R _CP (%) = 100 × (1 / d _He −1 / 2.26) / (V _OP + 1 / d _He )
Opening ratio R _OP (%) = 100 × V _OP / (V _OP + 1 / d _He )

  The interlayer distance of the porous carbon material was measured by the following procedure. 10 mass% standard Si (NIST 640d) was collected with respect to the porous carbon material and thoroughly mixed in a mortar to obtain a sample for X-ray diffraction measurement. The RINT2000 manufactured by Rigaku was used as the X-ray diffraction measurement apparatus. A Cu-Kα ray was used as the X-ray source, and the measurement output was measured by scanning 20 ° to 30 ° (2θ) by the 2θ / θ method with a tube voltage of 40 kV and a tube current of 40 mA. The carbon (002) peak in the vicinity of 23 to 26 degrees was corrected with the peak position of standard Si (111), and the distance between the (002) planes of the solid part was determined from the Bragg equation (d = λ / sin θc / 2). .

(B) Charge / Discharge Test Method of Negative Electrode Active Material The charge / discharge tests of the evaluation cells of Examples 1 to 7 and Comparative Examples 1 to 7 were performed to evaluate the charge / discharge characteristics. The method and result will be described.

  The charge / discharge test of the evaluation cell was performed in a constant temperature bath at 25 ° C. In the charge / discharge test, the test electrode containing the negative electrode active material was charged and the test was performed after discharging for 20 minutes. Charging / discharging was performed at a constant current at a current value of 0.05 mA per unit area of the negative electrode. Further, the end of charge was set to the time when the voltage reached 0V (charge end voltage: 0V). The end of discharge was the time when the voltage reached 2.0V (discharge end voltage: 2.0V). The value obtained by dividing the initial discharge capacity per unit weight of the negative electrode active material as the initial discharge capacity density per unit weight (mAh / g), and the value divided per unit volume of the negative electrode active material per unit volume (mAh / cc).

  The results of the charge / discharge test of the evaluation cells of Examples 1 to 7 and Comparative Examples 1 to 7 are shown in Table 1 together with the closed hole ratio, open hole ratio, and inter-surface distance of the negative electrode active material.

3. Discussion Examples 1-7 and Comparative Examples 1-7 all use a porous carbon material as a negative electrode active material. From the results shown in Table 1, depending on the structure of the porous carbon material, the negative electrode active material It has been found that the discharge capacity density as is greatly different.

  First, in Comparative Example 4, the initial discharge capacity density was significantly lower than those in Examples 1-7, Comparative Examples 1-3, and Comparative Examples 5-7. This is considered to be caused by the fact that the distance between the (002) planes of the solid portion is as small as 0.335 nm and does not function well as a sodium ion passage and a sodium ion occlusion / release site. Therefore, the distance between the (002) planes of the solid part is preferably larger than 0.335 nm, and is preferably 0.340 nm or more, which is the distance between the (002) planes of a standard carbon material. Further, in Comparative Examples 1 to 3 and Comparative Examples 5 to 7, since a high initial discharge capacity density was obtained compared to Comparative Example 4, the solid portion functions as a sodium ion passage and a sodium ion storage / release site. It is predicted that Therefore, it is preferable that the (002) plane distance of a solid part is 0.360 nm or more.

  Next, when the negative electrode active materials of Example 7 and Comparative Example 7 were compared, the discharge capacity density per weight showed the same value, but the discharge capacity density per volume differed greatly.

  In the negative electrode active material of Example 7, the distance between the (002) planes of the solid part is 0.383 nm, which functions well as a sodium ion passage and a sodium ion storage / release site, and has a large discharge capacity of 544 mAh / cc. Indicated. However, in the negative electrode active material of Comparative Example 7, the discharge capacity per volume was as small as 420 mAh / cc although the distance between the (002) planes of the solid part was almost the same value as 0.382 nm. . As a result, since the negative electrode active material of Comparative Example 7 has a large volume ratio of closed pores of 30.8%, the apparent density of the negative electrode active material decreases, and even if the discharge capacity density per weight is equal, This is probably because the discharge capacity density has decreased. From this result, it can be seen that the discharge capacity per unit volume can be increased if the distance between the (002) planes of the solid part is 0.340 nm or more and the volume ratio of the closed holes is 10% or less.

  Charging / discharging of the evaluation cells of Examples 1 to 7, Comparative Example 1, and Comparative Example 6 in which the distance between the (002) planes of the solid part is 0.340 nm or more and the volume ratio of closed holes is 10% or less FIG. 3 shows the relationship between the initial discharge capacity density per volume of the test and the volume ratio of the openings of the negative electrode active material. In each of the negative electrode active materials of Examples 1 to 7, the distance between the (002) planes of the solid part was 0.340 nm or more, the volume ratio of closed holes was 10% or less, and the volume ratio of open holes was 50%. It is as follows. The initial discharge capacity per unit volume was 448 mAh / cc or more. From this result, if the distance between the (002) planes of the solid part is 0.340 nm or more, the volume ratio of closed holes is 10% or less, and the volume ratio of openings is 50% or less, the conventional hard carbon carbon material It can be seen that a high capacity exceeding can be obtained. Furthermore, in Examples 1 to 3 and Example 7, a higher discharge capacity density per volume was obtained than in Examples 4 to 6. This is presumably because the irreversible reaction occurring on the surface of the aperture decreased due to the smaller volume ratio of the aperture. Therefore, it can be seen that the volume ratio of the holes is preferably 20% or less.

  In the negative electrode active materials of Comparative Example 1 and Comparative Example 6, the distance between the (002) planes of the solid part is sufficiently large as 0.388 nm and 0.370 nm, and the closed hole ratio is 6.4% and 0.8%. Therefore, although the occlusion site of sodium ions can be sufficiently secured and the apparent density is large, the ratio of pores is as large as 51.3% and 72.5%. The initial discharge capacity density per volume was as small as 343 mAh / cc and 215 mAh / cc.

  In the negative electrode active materials of Comparative Example 2, Comparative Example 3 and Comparative Example 5, the distances between (002) planes of the solid part are sufficiently large as 0.379 nm, 0.363 and 0.379 nm, and the aperture ratio is 2. Since 4%, 4.1% and 11.0 are small, a sufficient occlusion site for sodium ions can be secured, and the negative reversible reaction occurring on the surface of the pores is suppressed, but the pore closing ratio is 11.0%. Since it was large as 35.9% and 27.2%, the apparent density was small, and the initial discharge capacity density was as small as 448 mAh / cc, 361 mAh / cc, and 416 mAh / cc.

  From the above results, it is a porous carbon material composed of an open hole communicating to the surface, a closed hole not communicating to the surface, and a solid part, and the (002) interplanar distance of the solid part is greater than 0.334 nm, A carbon material having a closed hole volume ratio of 10% or less and an open hole volume ratio of 50% or less has a large discharge capacity density per volume as a negative electrode active material for sodium ion secondary batteries of 448 mAh / cc or more. It was confirmed to have.

  Moreover, it has shown that the sodium ion secondary battery using the negative electrode active material with a large discharge capacity density per volume can provide a storage battery with a large capacity density per volume, that is, a compact.

  In the above embodiment, the negative electrode active material in which at least some of the holes are open or closed has been described. However, it is preferable that the volume ratio of the open holes or closed holes is as small as possible, and does not exclude negative electrode active materials that do not have open holes or closed holes. It is considered that the negative electrode active material having no open or closed holes can be realized, for example, by appropriately selecting heat treatment conditions.

  The sodium ion secondary battery of one embodiment of the present invention includes a power source for portable electronic devices and the like; a power storage device for power leveling used in combination with power generation facilities such as thermal power generation, wind power generation, and fuel cell power generation; It can be suitably used as a power source for an emergency power storage system for an apartment house, a midnight power storage system, an uninterruptible power supply, a transportation device such as an electric vehicle, a hybrid vehicle, and a plug-in hybrid vehicle.

DESCRIPTION OF SYMBOLS 10 Negative electrode active material 11 Surface 12 Open hole 13 Closed hole 14 Solid part 15 Negative electrode mixture layer 21 Positive electrode collector 22 Positive electrode mixture layer 23 Positive electrode 24 Negative electrode collector 25 Negative electrode mixture layer 26 Negative electrode 27 Separator 28 Exterior

Claims (12)

A porous carbon material having a plurality of apertures communicating with the surface, a plurality of closed pores not communicating with the surface, and a solid portion made of a carbon material;
The (002) plane distance of the solid part is 0.340 nm or more and 0.410 nm or less,
The volume ratio of the plurality of closed holes to the volume sum of the plurality of openings, the plurality of closed holes, and the solid portion is 0% or more and 10% or less,
A negative electrode active material for a sodium ion secondary battery, wherein a volume ratio of the plurality of openings to the volume sum of the plurality of openings, the plurality of closed holes, and the solid portion is 0% or more and 50% or less.   2. The negative electrode active material for a sodium ion secondary battery according to claim 1, wherein a distance between the (002) planes of the solid part is 0.36 nm or more.   2. The negative electrode active material for a sodium ion secondary battery according to claim 1, wherein a volume ratio of the plurality of openings to the volume sum of the plurality of openings, the plurality of closed holes, and the solid portion is 20% or less. A negative electrode comprising the negative electrode active material according to claim 1;
A positive electrode containing a positive electrode active material capable of occluding and releasing sodium ions;
A sodium ion secondary battery comprising an electrolyte containing sodium ions. A step of preparing an activated carbon material;
Including a step of obtaining a porous carbon material by heat-treating the activated carbon material in an inert atmosphere,
The porous carbon material has a plurality of openings communicating with the surface, a plurality of closed holes not communicating with the surface, and a solid portion made of a carbon material,
The distance between the (002) planes of carbon in at least a part of the solid part is 0.340 nm or more and 0.410 nm or less,
The volume ratio of the plurality of closed holes to the volume sum of the plurality of openings, the plurality of closed holes, and the solid portion is 0% or more and 10% or less,
The method for producing a negative electrode active material for a sodium ion secondary battery, wherein a volume ratio of the plurality of openings to the volume sum of the plurality of openings, the plurality of closed holes, and the solid portion is 0% or more and 50% or less. Preparing an organic material as a carbon source;
A step of obtaining a porous carbon material by heat-treating the organic material in an inert atmosphere,
The organic material is a cellulosic resin,
After performing the heat treatment, the porous carbon material is not pulverized,
The porous carbon material has a plurality of openings communicating with the surface, a plurality of closed holes not communicating with the surface, and a solid portion made of a carbon material,
The distance between the (002) planes of carbon in at least a part of the solid part is 0.340 nm or more and 0.410 nm or less,
The volume ratio of the plurality of closed holes to the volume sum of the plurality of openings, the plurality of closed holes, and the solid portion is 0% or more and 10% or less,
The method for producing a negative electrode active material for a sodium ion secondary battery, wherein a volume ratio of the plurality of openings to the volume sum of the plurality of openings, the plurality of closed holes, and the solid portion is 0% or more and 50% or less. Preparing an organic material as a carbon source;
A step of obtaining a porous carbon material by heat-treating the organic material in an inert atmosphere,
The organic material is a phenolic resin,
After performing the heat treatment, the porous carbon material is not pulverized,
The porous carbon material has a plurality of openings communicating with the surface, a plurality of closed holes not communicating with the surface, and a solid portion made of a carbon material,
The distance between the (002) planes of carbon in at least a part of the solid part is 0.340 nm or more and 0.410 nm or less,
The volume ratio of the plurality of closed holes to the volume sum of the plurality of openings, the plurality of closed holes, and the solid portion is 0% or more and 10% or less,
A method for producing a negative electrode active material for a sodium ion secondary battery, wherein a volume ratio of the plurality of openings to the volume sum of the plurality of openings, the plurality of closed holes, and the solid portion is 0% or more and 50% or less. Preparing an organic material as a carbon source;
A step of obtaining a porous carbon material by heat-treating the organic material in an inert atmosphere,
The organic material is phenolphthalein,
The temperature of the heat treatment is 1000 ° C. or higher and 1300 ° C. or lower,
After performing the heat treatment, the porous carbon material is not pulverized,
The porous carbon material has a plurality of openings communicating with the surface, a plurality of closed holes not communicating with the surface, and a solid portion made of a carbon material,
The distance between the (002) planes of carbon in at least a part of the solid part is 0.340 nm or more and 0.410 nm or less,
The volume ratio of the plurality of closed holes to the volume sum of the plurality of openings, the plurality of closed holes, and the solid portion is 0% or more and 10% or less,
A method for producing a negative electrode active material for a sodium ion secondary battery, wherein a volume ratio of the plurality of openings to the volume sum of the plurality of openings, the plurality of closed holes, and the solid portion is 0% or more and 50% or less.   The temperature of the said heat processing is 1100 degreeC or more and 1300 degrees C or less, The manufacturing method of the negative electrode active material for sodium ion secondary batteries of Claim 6.   The method for producing a negative electrode active material for a sodium ion secondary battery according to claim 7, wherein the temperature of the heat treatment is 1000 ° C. or higher and 1300 ° C. or lower.   The method for producing a negative electrode active material for a sodium ion secondary battery according to claim 5, wherein the temperature of the heat treatment is 1600 ° C. or higher and 2500 ° C. or lower. Before the step of heat-treating the organic material,
Carbonizing the organic material to obtain a carbide;
And crushing the carbide.
The method for producing a negative electrode active material for a sodium ion secondary battery according to any one of claims 6 to 10 , wherein the pulverized carbide is heat-treated in the step of heat-treating the organic material.

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